Rechargeable batteries for use with electronic devices such as laptop PCs are often provided in a pack of battery cells. The use of a pack of cells, rather than a single cell, can provide higher voltage delivery or a greater capacity through increased amp-hours. Individual cells included in such a pack, however, have a tendency to accept a charge at different rates. Over a series of charge cycles, an individual cell can deviate significantly from the charge capacity of other cells in the pack.
Certain cell types, such as lithium-ion cells, can also be hazardous if an attempt is made to charge them significantly above or below their normal charge range. For this reason, charge boundary protective circuitry must be included in such packs to guard against this type of condition. Since, however, all cells in a pack are charged and drained simultaneously, such circuitry tends to stop charging before all cells have a full charge, and tends to stop discharging a pack before all cells are fully discharged.
A typical charger protection circuit employs four threshold voltage values which serve to keep the cells charged within a safe range. An over-voltage threshold V(ov) sets the maximum allowable charge for a cell, at which point the protection circuit will prevent further charging. An under-voltage threshold V(uv) sets the minimum charge which a cell may not fall below, at which point no current is allowed to flow. An over-voltage-to-normal V(ovn) threshold is slightly below the over-voltage threshold, and is used to set a protection circuit state machine to provide hysteresis in transitioning between various states in the charge cycle. An under-voltage-to-normal threshold V(uvn) is slightly above the under-voltage threshold, and is likewise used to set a state machine.
Therefore, during charging of the pack, the first cell to reach V(ov) stops charging of all cells, and sets the state machine such that the protection circuit will not allow further charging until all cells fall below V(ovn). In other words, the cell must discharge by a predetermined amount set by V(ovn) until further charging will be permitted. Similarly, the first cell to discharge down to V(uv) will shut off the pack, and sets the state machine such that no current will be permitted to flow until all cells are above V(uvn), and thus further discharge will be prevented until the cells are charged by a certain level set by V(uvn).
Further, the chargeable material in many rechargeable cells undergoes a conditioning in the first few charge cycles. Such conditioning preferably entails several full charge and discharge cycles to utilize all chargeable (charge retaining) material of the cell. The level of charge achieved by this conditioning tends to affect the long term ability of the chargeable material to accept and sustain a charge. Therefore, a less than full charge or less than full discharge can cause the cell to be permanently attributed this lesser charge capacity range.
As indicated above, a typical prior art protection circuit turns off pack charging when any cell in a pack attains the overvoltage threshold V(ov) and turns off pack discharge by shutting down the pack after the first cell has discharged down to the undervoltage threshold V(uv). This type of circuit results in possibly only one cell being fully charged, and only one cell being fully discharged, in each cycle. Therefore, the most deviant cells tend to dictate the performance of the pack as a whole. In a pack with a large number of cells, cells with an intermediate charge capacity are likely to never be fully charged or discharged, as a stronger or weaker cell triggers the charging limits first. As a result, such cells will not become conditioned for an optimal charging capacity.
For these reasons, such multi-cell battery packs could benefit from a charge balancing circuit. Such a circuit would attempt to selectively turn off cells which have reached full voltage during the charging process, while allowing others which have not yet attained full potential to continue charging.
These charge balancing circuits attempt to balance the charge by individually monitoring the voltage of each cell, rather than by using the first cell to reach V(ovn) and V(uvn) as the charge start/stop triggers. Such circuits, however, tend to require a quantity of cell specific components to individually monitor and control the charge level of each cell. As a result, the complexity and number of components rises proportionally with the number of cells, increasing production and maintenance costs of the pack.
It would be beneficial to devise a charge balancing circuit with a minimal set of components which can be applied to a plurality of cells in a pack.